Solvent- and Co-Catalyst-Free Cycloaddition of Carbon Dioxide and Epoxides Catalyzed by Recyclable Bifunctional Niobium Complexes

CO2, as a cheap and abundant renewable C1 resource, can be used to synthesize high value-added chemicals. In this paper, a series of bifunctional metallic niobium complexes were synthesized and their structures were characterized by IR, NMR and elemental analysis. All of these complexes have been proved to be efficient catalysts for the coupling reaction of CO2 and epoxides to obtain cyclic carbonates under solvent- and co-catalyst-free conditions. By using CO2 and propylene oxide as a model reaction, the optimal reaction conditions were systematically screened as: 100 °C, 1 MPa, 2 h, ratio of catalyst to alkylene oxide 1:100. Under the optimal reaction conditions, the bifunctional niobium catalysts can efficiently catalyze the coupling reaction with high yield and excellent selectivity (maximum yield of >99% at high pressure and 96.8% at atmospheric pressure). Moreover, this series of catalysts can also catalyze the coupling reaction at atmospheric pressure and most of them showed high conversion of epoxide. The catalysts have good substrate suitability and are also applicable to a variety of epoxides including diepoxides and good catalytic performances were achieved for producing the corresponding cyclic carbonates in most cases. Furthermore, the catalysts can be easily recovered by simple filtration and reused for at least five times without obvious loss of catalytic activity and selectivity. Kinetic studies were carried out preliminarily for the bifunctional niobium complexes with different halogen ions (3a(Cl−), 3b(Br−), 3c(I−)) and the formation activation energies (Ea) of cyclic carbonates were obtained. The order of apparent activation energy Ea is 3a (96.2 kJ/mol) > 3b (68.2 kJ/mol) > 3c (37.4 kJ/mol). Finally, a possible reaction mechanism is proposed.


Introduction
In the last one to two centuries, with the large-scale application of fossil energy sources such as coal, oil and natural gas in countries around the world, a large amount of greenhouse gases has been released and the CO 2 content in the atmosphere has increased, which has led to an increase in global temperature and frequent global extreme weather [1]. Meanwhile, CO 2 , as an abundant non-toxic, cheap and easily available C1 resource, can be used to synthesize a series of industrial products with high added value [2], which provides an idea to alleviate environmental problems. CO 2 can be used as a building block to construct C-C, C-O, and C-N bonds for the synthesis of methanol [3], cyclic carbonates [4], oxazolidinones [5], and amides [6], which are important chemical intermediates and pharmaceutical intermediates. Among them, the construction of C-O bonds is a focus as well as a hot spot of current research.

Experimental Procedure for the Cycloaddition of Carbon Dioxide and Epoxides
Reaction under high pressure: The quantitative epoxy compound and bifunctional niobium complex were added into a stainless steel reactor with a magnetic stir, then the reactor was sealed. CO 2 was pressurized into the reactor to replace the gas three times before it was immersed into the oil bath with pre-set temperature for 20 min. The reaction started under stable CO 2 pressure in the reactor. When the preset time was reached, the reaction vessel was cooled quickly with ice water to release the pressure slowly. After exhausting the gas, a small amount of the mixture was taken for 1 H NMR characterization to calculate the yield and selectivity.
Reaction under atmospheric pressure: Under the protection of CO 2 , a quantitative amount of bifunctional niobium complex and stir bar were added into the Schlenk bottle connecting with a CO 2 balloon sealed with rubber cap. Then, a quantitative amount of epoxide compound was injected into the bottle with a syringe. The reaction system was preheated in a constant temperature oil bath for 20 min and then the reaction was started. When the required reaction time was reached, the reactor was cooled quickly with ice water to release the pressure slowly, and then a small amount of the mixture was taken with a syringe for 1 H NMR analysis and the yield and selectivity were calculated.

Optimization of Reaction Conditions
The synthetic route of cyclic carbonate is shown in Figure 1. The effect of catalyst type, catalyst dosage, temperature, time and CO 2 pressure on the reaction was investigated systematically, and ultimately the optimum reaction conditions were explored, under which a variety of epoxides were catalyzed to investigate the catalytic efficiency and substrate suitability of the catalytic system.

Effect of Catalyst Type on Catalytic Activity
Firstly, the effect of anion halogen ions on catalyst activity was tested under the conditions of 100 °C, 1 MPa, 2 h, catalyst:epoxide = 1:100 (Table 1). It can be seen that there is no propylene carbonate (PC) produced without the catalyst. Ionic liquids have been proved as efficient catalysts, so the catalytic activities of ligands 2a-2e have also been performed and good results were obtained (Entries 2-6). However, the recyclability of these ligands is worse compared with their metal complexes. The catalytic activities of three niobium complexes 3a-3c were proved as efficient catalysts showing almost the same activity within 2 h. To clarify the catalytic performance of 3a-3c, shorter time reactions of 1 h have been investigated and there are still no big differences for their catalytic activities as shown in Table 1 (Entries 13-15). Due to the lower yields for the preparation of catalysts with Br − and I − ions, the catalyst 3a containing Clions were chosen for the following studies. The effect of the alkyl substituent on the imidazole moiety of catalyst activity was then investigated (Entries 7,10,11). The length of the alkyl chain showed a relatively significant effect on the catalyst activity. The best yields were obtained when the substituent was methyl (3a). The reason for this occurrence may be due to the effect of spatial hindrance, the greater the spatial hindrance, the lower the catalytic activity. So 3a was chosen as the optimal catalyst for the coupling reaction of CO2 and epoxide.

Effect of Catalyst Type on Catalytic Activity
Firstly, the effect of anion halogen ions on catalyst activity was tested under the conditions of 100 • C, 1 MPa, 2 h, catalyst:epoxide = 1:100 (Table 1). It can be seen that there is no propylene carbonate (PC) produced without the catalyst. Ionic liquids have been proved as efficient catalysts, so the catalytic activities of ligands 2a-2e have also been performed and good results were obtained (Entries 2-6). However, the recyclability of these ligands is worse compared with their metal complexes. The catalytic activities of three niobium complexes 3a-3c were proved as efficient catalysts showing almost the same activity within 2 h. To clarify the catalytic performance of 3a-3c, shorter time reactions of 1 h have been investigated and there are still no big differences for their catalytic activities as shown in Table 1 (Entries 13-15). Due to the lower yields for the preparation of catalysts with Br − and I − ions, the catalyst 3a containing Clions were chosen for the following studies. The effect of the alkyl substituent on the imidazole moiety of catalyst activity was then investigated (Entries 7,10,11). The length of the alkyl chain showed a relatively Materials 2023, 16, 3531 5 of 12 significant effect on the catalyst activity. The best yields were obtained when the substituent was methyl (3a). The reason for this occurrence may be due to the effect of spatial hindrance, the greater the spatial hindrance, the lower the catalytic activity. So 3a was chosen as the optimal catalyst for the coupling reaction of CO 2 and epoxide.

Effect of Reaction Parameters on Catalytic Activity
After determining the type of catalyst, the coupling reaction of propylene oxide and carbon dioxide was used as a model reaction to explore the effects of temperature, CO 2 pressure, time, and catalyst dosage on the reaction activity ( Figure 2).
As shown in Figure 2a, low temperature resulted in lower activity. When the temperature gradually increases, the product yield continues to increase. Although the yield is a little better at 120 • C than that obtained at 100 • C, the increase is not obvious. The cycloaddition reaction of CO 2 with epoxide is exothermic, so from the viewpoint of thermodynamic equilibrium, too high temperature will hinder the formation of cyclic carbonate [28,29]. In addition, high temperature also leads to the polymerization of cyclic carbonate, which reduces the catalytic efficiency [30]. Therefore, 100 • C is selected as the optimal reaction temperature.
The effect of CO 2 pressure on the catalytic activity is shown in Figure 2b. The product yield showed a trend of first rising and then decreasing with the CO 2 pressure and the highest activity was obtained at 1 MPa. The pressure is increased first because of the increase in CO 2 concentration involved in the reaction. Therefore, the yield showed an upward trend at the initial stage. When the pressure increases to a certain value, too high CO 2 pressure would decrease the propylene oxide (PO) concentration in the vicinity of the catalyst to lower PC yield. These opposite factors' competition gave rise to an optimal pressure of 1 MPa for the best PC yields [31,32].
Time is another indispensable factor and its effect is shown in Figure 2c. The yield of cyclic carbonate increases with time. At the early stage, the yield increases sharply and almost linearly, but the yield hardly increases when the time exceeds 2 h, so it is appropriate to choose the optimal reaction time as 2 h.
In addition to these factors, the influence of the amount of catalyst on the reaction activity is also important. As shown in Figure 2d, when the amount of catalyst is at a lower level, the product yield is relatively low too. When the ratio of the amount of substrate to catalyst reaches 1:100, the product yield reaches the maximum 96%. However, the increase in product yield is not obvious when the amount of catalyst continues to increase. So, the ratio of catalyst to alkylene oxide 1:100 is selected as the optimal value. Materials 2023, 16, x FOR PEER REVIEW 6 of 13 As shown in Figure 2a, low temperature resulted in lower activity. When the temperature gradually increases, the product yield continues to increase. Although the yield is a little better at 120 °C than that obtained at 100 °C, the increase is not obvious. The cycloaddition reaction of CO2 with epoxide is exothermic, so from the viewpoint of thermodynamic equilibrium, too high temperature will hinder the formation of cyclic carbonate [28,29]. In addition, high temperature also leads to the polymerization of cyclic carbonate, which reduces the catalytic efficiency [30]. Therefore, 100 °C is selected as the optimal reaction temperature.
The effect of CO2 pressure on the catalytic activity is shown in Figure 2b. The product yield showed a trend of first rising and then decreasing with the CO2 pressure and the highest activity was obtained at 1 MPa. The pressure is increased first because of the increase in CO2 concentration involved in the reaction. Therefore, the yield showed an upward trend at the initial stage. When the pressure increases to a certain value, too high CO2 pressure would decrease the propylene oxide (PO) concentration in the vicinity of the catalyst to lower PC yield. These opposite factors' competition gave rise to an optimal pressure of 1 MPa for the best PC yields [31,32].
Time is another indispensable factor and its effect is shown in Figure 2c. The yield of cyclic carbonate increases with time. At the early stage, the yield increases sharply and almost linearly, but the yield hardly increases when the time exceeds 2 h, so it is appropriate to choose the optimal reaction time as 2 h.
In addition to these factors, the influence of the amount of catalyst on the reaction activity is also important. As shown in Figure 2d, when the amount of catalyst is at a In summary, the optimal conditions for the bifunctional niobium complex for the cycloaddition of CO 2 and epoxide were screened as: reaction temperature of 100 • C, carbon dioxide pressure of 1 MPa, reaction time of 2 h and catalyst to epoxide ratio of 1:100.

Applicability of Substrates
To investigate the suitability of the catalytic system for more substrates expansion, various epoxides were tested for the coupling reaction both at high and atmospheric pressure and the results are shown in Table 2.
Under optimal reaction conditions (100 • C, 1 MPa, 2 h, 1:100 catalyst to epoxide ratio), the bifunctional niobium complex can efficiently catalyze the cycloaddition reactions of a wide range of epoxides with CO 2 . The yields of cyclic carbonate for epoxides with relatively low spatial hindrance, such as epichlorohydrin, 2-(isopropoxymethyl)oxirane, 2-phenyloxirane and 2-butyloxirane, the corresponding yields of cyclic carbonates are 100%, 96.4%, 90.9%, and 74.8%, respectively (entries 1-4), but for epoxides with bigger steric hindrance, such as cyclohexene oxide and 2,2-dimethyloxirane, the reaction was extended to 12 h with only moderate yields (Table 2, entries 5,6). It is worth noting that this catalyst is also suitable for bis-epoxides, which can be obtained in excellent yields (entries 7,8). The bicyclic carbonate synthesized from the bis-epoxides has an important role in industry, which is a feedstock for the reaction with polyfunctional primary amines to produce non-isocyanate polyurethanes (NIPUs). The present catalyst is also suitable for substrates of the glycidyl ether family, showing excellent catalytic activity (entries 9-12). achieved under reaction conditions (entries 7-8). For glycidyl ether co for phenyl glycidyl ether, excellent yields can be achieved within 11 h. A ethers required longer reaction times to achieve excellent yields (entries In summary, the catalytic system has good substrate suitability and reaction of a variety of epoxides with CO2 to form the corresponding under both high pressure and atmospheric conditions with satisfactory achieved under reaction conditions (entries 7-8). For glycidyl ether compounds, ex for phenyl glycidyl ether, excellent yields can be achieved within 11 h. All other glyc ethers required longer reaction times to achieve excellent yields (entries 9-12). In summary, the catalytic system has good substrate suitability and can catalyze reaction of a variety of epoxides with CO2 to form the corresponding cyclic carbon under both high pressure and atmospheric conditions with satisfactory results. achieved under reaction conditions (entries 7-8). For glycidyl ether c for phenyl glycidyl ether, excellent yields can be achieved within 11 h ethers required longer reaction times to achieve excellent yields (entrie In summary, the catalytic system has good substrate suitability an reaction of a variety of epoxides with CO2 to form the corresponding under both high pressure and atmospheric conditions with satisfactory achieved under reaction conditions (entries 7-8). For glycidyl ether compounds, e for phenyl glycidyl ether, excellent yields can be achieved within 11 h. All other gl ethers required longer reaction times to achieve excellent yields (entries 9-12). In summary, the catalytic system has good substrate suitability and can cataly reaction of a variety of epoxides with CO2 to form the corresponding cyclic carbo under both high pressure and atmospheric conditions with satisfactory results.

Kinetic Study
The kinetics of the carbon dioxide cycloaddition reaction catalyze niobium complexes was also investigated in detail. The cycloaddition o ether (BGE) and CO2 was selected as model reaction at atmospheric kinetic behavior of bifunctional niobium complexes containing differ was studied in the temperature range of 353-413 K and the reaction tim (please refer to the Supplementary Materials). The apparent activation bifunctional niobium complexes with different halogen ions are shown order of apparent activation energy Ea is 3a (96.2 kJ/mol) > 3b (68.2 kJ/mol) (Figure 3), which is consistent with the yields of 96%, 97% a carbonate formation from PC and CO2 catalyzed by 3a, 3b, and 3c, res

Kinetic Study
The kinetics of the carbon dioxide cycloaddition reaction catalyzed by bifuncti niobium complexes was also investigated in detail. The cycloaddition of n-butyl glyc ether (BGE) and CO2 was selected as model reaction at atmospheric pressure, and kinetic behavior of bifunctional niobium complexes containing different halogen was studied in the temperature range of 353-413 K and the reaction time range of 2 (please refer to the Supplementary Materials). The apparent activation energy Ea of bifunctional niobium complexes with different halogen ions are shown in Figure 3. order of apparent activation energy Ea is 3a (96.2 kJ/mol) > 3b (68.2 kJ/mol) >3c ( kJ/mol) (Figure 3), which is consistent with the yields of 96%, 97% and 99% of c carbonate formation from PC and CO2 catalyzed by 3a, 3b, and 3c, respectively. Th

Kinetic Study
The kinetics of the carbon dioxide cycloaddition reaction catalyze niobium complexes was also investigated in detail. The cycloaddition o ether (BGE) and CO2 was selected as model reaction at atmospheric p kinetic behavior of bifunctional niobium complexes containing differ was studied in the temperature range of 353-413 K and the reaction tim (please refer to the Supplementary Materials). The apparent activation bifunctional niobium complexes with different halogen ions are shown order of apparent activation energy Ea is 3a (96.2 kJ/mol) > 3b (68.2 kJ/mol) (Figure 3), which is consistent with the yields of 96%, 97% a carbonate formation from PC and CO2 catalyzed by 3a, 3b, and 3c, res

Kinetic Study
The kinetics of the carbon dioxide cycloaddition reaction catalyzed by bifunct niobium complexes was also investigated in detail. The cycloaddition of n-butyl gly ether (BGE) and CO2 was selected as model reaction at atmospheric pressure, and kinetic behavior of bifunctional niobium complexes containing different halogen was studied in the temperature range of 353-413 K and the reaction time range of 2 (please refer to the Supplementary Materials). The apparent activation energy Ea o bifunctional niobium complexes with different halogen ions are shown in Figure 3. order of apparent activation energy Ea is 3a (96.2 kJ/mol) > 3b (68.2 kJ/mol) >3c kJ/mol) (Figure 3), which is consistent with the yields of 96%, 97% and 99% of c carbonate formation from PC and CO2 catalyzed by 3a, 3b, and 3c, respectively. Th

Kinetic Study
The kinetics of the carbon dioxide cycloaddition reaction catalyze niobium complexes was also investigated in detail. The cycloaddition o ether (BGE) and CO2 was selected as model reaction at atmospheric kinetic behavior of bifunctional niobium complexes containing differ was studied in the temperature range of 353-413 K and the reaction tim (please refer to the Supplementary Materials). The apparent activation bifunctional niobium complexes with different halogen ions are shown order of apparent activation energy Ea is 3a (96.2 kJ/mol) > 3b (68.2 kJ/mol) (Figure 3), which is consistent with the yields of 96%, 97% a carbonate formation from PC and CO2 catalyzed by 3a, 3b, and 3c, re

Kinetic Study
The kinetics of the carbon dioxide cycloaddition reaction catalyzed by bifuncti niobium complexes was also investigated in detail. The cycloaddition of n-butyl glyc ether (BGE) and CO2 was selected as model reaction at atmospheric pressure, and kinetic behavior of bifunctional niobium complexes containing different halogen was studied in the temperature range of 353-413 K and the reaction time range of 2 (please refer to the Supplementary Materials). The apparent activation energy Ea of bifunctional niobium complexes with different halogen ions are shown in Figure 3. order of apparent activation energy Ea is 3a (96.2 kJ/mol) > 3b (68.2 kJ/mol) >3c ( kJ/mol) (Figure 3), which is consistent with the yields of 96%, 97% and 99% of cy carbonate formation from PC and CO2 catalyzed by 3a, 3b, and 3c, respectively. Th

Kinetic Study
The kinetics of the carbon dioxide cycloaddition reaction catalyzed niobium complexes was also investigated in detail. The cycloaddition of ether (BGE) and CO2 was selected as model reaction at atmospheric p kinetic behavior of bifunctional niobium complexes containing differe was studied in the temperature range of 353-413 K and the reaction tim (please refer to the Supplementary Materials). The apparent activation bifunctional niobium complexes with different halogen ions are shown order of apparent activation energy Ea is 3a (96.2 kJ/mol) > 3b (68.2 k kJ/mol) (Figure 3), which is consistent with the yields of 96%, 97% an carbonate formation from PC and CO2 catalyzed by 3a, 3b, and 3c, resp Materials 2023, 16

Kinetic Study
The kinetics of the carbon dioxide cycloaddition reaction catalyzed by bifuncti niobium complexes was also investigated in detail. The cycloaddition of n-butyl glyc ether (BGE) and CO2 was selected as model reaction at atmospheric pressure, and kinetic behavior of bifunctional niobium complexes containing different halogen was studied in the temperature range of 353-413 K and the reaction time range of 2 (please refer to the Supplementary Materials). The apparent activation energy Ea of bifunctional niobium complexes with different halogen ions are shown in Figure 3. order of apparent activation energy Ea is 3a (96.2 kJ/mol) > 3b (68.2 kJ/mol) >3c ( kJ/mol) (Figure 3), which is consistent with the yields of 96%, 97% and 99% of cy carbonate formation from PC and CO2 catalyzed by 3a, 3b, and 3c, respectively. Th

Kinetic Study
The kinetics of the carbon dioxide cycloaddition reaction catal niobium complexes was also investigated in detail. The cycloadditio ether (BGE) and CO2 was selected as model reaction at atmospher kinetic behavior of bifunctional niobium complexes containing di was studied in the temperature range of 353-413 K and the reaction (please refer to the Supplementary Materials). The apparent activat bifunctional niobium complexes with different halogen ions are sho order of apparent activation energy Ea is 3a (96.2 kJ/mol) > 3b (68 kJ/mol) (Figure 3), which is consistent with the yields of 96%, 97% carbonate formation from PC and CO2 catalyzed by 3a, 3b, and 3c,

Kinetic Study
The kinetics of the carbon dioxide cycloaddition reaction catalyzed by bifun niobium complexes was also investigated in detail. The cycloaddition of n-butyl g ether (BGE) and CO2 was selected as model reaction at atmospheric pressure, a kinetic behavior of bifunctional niobium complexes containing different haloge was studied in the temperature range of 353-413 K and the reaction time range o (please refer to the Supplementary Materials). The apparent activation energy Ea bifunctional niobium complexes with different halogen ions are shown in Figure  order of apparent activation energy Ea is 3a (96.2 kJ/mol) > 3b (68.2 kJ/mol) >3 kJ/mol) (Figure 3), which is consistent with the yields of 96%, 97% and 99% of carbonate formation from PC and CO2 catalyzed by 3a, 3b, and 3c, respectively.

Kinetic Study
The kinetics of the carbon dioxide cycloaddition reaction cataly niobium complexes was also investigated in detail. The cycloaddition ether (BGE) and CO2 was selected as model reaction at atmospher kinetic behavior of bifunctional niobium complexes containing dif was studied in the temperature range of 353-413 K and the reaction (please refer to the Supplementary Materials). The apparent activat bifunctional niobium complexes with different halogen ions are sho order of apparent activation energy Ea is 3a (96.2 kJ/mol) > 3b (68 kJ/mol) (Figure 3)

Kinetic Study
The kinetics of the carbon dioxide cycloaddition reaction catalyzed by bifu niobium complexes was also investigated in detail. The cycloaddition of n-butyl ether (BGE) and CO2 was selected as model reaction at atmospheric pressure, kinetic behavior of bifunctional niobium complexes containing different halog was studied in the temperature range of 353-413 K and the reaction time range (please refer to the Supplementary Materials). The apparent activation energy E bifunctional niobium complexes with different halogen ions are shown in Figur order of apparent activation energy Ea is 3a (96.2 kJ/mol) > 3b (68.2 kJ/mol) > kJ/mol) (Figure 3), which is consistent with the yields of 96%, 97% and 99% o carbonate formation from PC and CO2 catalyzed by 3a, 3b, and 3c, respectively

Kinetic Study
The kinetics of the carbon dioxide cycloaddition reaction cataly niobium complexes was also investigated in detail. The cycloadditio ether (BGE) and CO2 was selected as model reaction at atmospher kinetic behavior of bifunctional niobium complexes containing dif was studied in the temperature range of 353-413 K and the reaction (please refer to the Supplementary Materials). The apparent activat bifunctional niobium complexes with different halogen ions are sho order of apparent activation energy Ea is 3a (96.2 kJ/mol) > 3b (68 kJ/mol) (Figure 3), which is consistent with the yields of 96%, 97% carbonate formation from PC and CO2 catalyzed by 3a, 3b, and 3c, attributed to the leaving ability of the contained halogen ions (Cl − < B

Kinetic Study
The kinetics of the carbon dioxide cycloaddition reaction catalyzed by bifun niobium complexes was also investigated in detail. The cycloaddition of n-butyl g ether (BGE) and CO2 was selected as model reaction at atmospheric pressure, a kinetic behavior of bifunctional niobium complexes containing different haloge was studied in the temperature range of 353-413 K and the reaction time range o (please refer to the Supplementary Materials). The apparent activation energy Ea bifunctional niobium complexes with different halogen ions are shown in Figure  order of apparent activation energy Ea is 3a (96.2 kJ/mol) > 3b (68.2 kJ/mol) >3 kJ/mol) (Figure 3), which is consistent with the yields of 96%, 97% and 99% of carbonate formation from PC and CO2 catalyzed by 3a, 3b, and 3c, respectively.

Kinetic Study
The kinetics of the carbon dioxide cycloaddition reaction catalyz niobium complexes was also investigated in detail. The cycloaddition ether (BGE) and CO2 was selected as model reaction at atmospheric kinetic behavior of bifunctional niobium complexes containing diff was studied in the temperature range of 353-413 K and the reaction t (please refer to the Supplementary Materials). The apparent activatio bifunctional niobium complexes with different halogen ions are show order of apparent activation energy Ea is 3a (96.2 kJ/mol) > 3b (68. kJ/mol) (Figure 3), which is consistent with the yields of 96%, 97% carbonate formation from PC and CO2 catalyzed by 3a, 3b, and 3c, r attributed to the leaving ability of the contained halogen ions (Cl − < Br

Kinetic Study
The kinetics of the carbon dioxide cycloaddition reaction catalyzed by bifunct niobium complexes was also investigated in detail. The cycloaddition of n-butyl gly ether (BGE) and CO2 was selected as model reaction at atmospheric pressure, and kinetic behavior of bifunctional niobium complexes containing different halogen was studied in the temperature range of 353-413 K and the reaction time range of 2 (please refer to the Supplementary Materials). The apparent activation energy Ea o bifunctional niobium complexes with different halogen ions are shown in Figure 3 order of apparent activation energy Ea is 3a (96.2 kJ/mol) > 3b (68.2 kJ/mol) >3c kJ/mol) (Figure 3), which is consistent with the yields of 96%, 97% and 99% of c carbonate formation from PC and CO2 catalyzed by 3a, 3b, and 3c, respectively. Th

Kinetic Study
The kinetics of the carbon dioxide cycloaddition reaction catalyz niobium complexes was also investigated in detail. The cycloaddition ether (BGE) and CO2 was selected as model reaction at atmospheric kinetic behavior of bifunctional niobium complexes containing diffe was studied in the temperature range of 353-413 K and the reaction ti (please refer to the Supplementary Materials). The apparent activatio bifunctional niobium complexes with different halogen ions are show order of apparent activation energy Ea is 3a (96.2 kJ/mol) > 3b (68.2 kJ/mol) (Figure 3), which is consistent with the yields of 96%, 97% carbonate formation from PC and CO2 catalyzed by 3a, 3b, and 3c, re attributed to the leaving ability of the contained halogen ions (Cl − < Br − Materials 2023, 16

Kinetic Study
The kinetics of the carbon dioxide cycloaddition reaction catalyzed by bifunct niobium complexes was also investigated in detail. The cycloaddition of n-butyl gly ether (BGE) and CO2 was selected as model reaction at atmospheric pressure, an kinetic behavior of bifunctional niobium complexes containing different halogen was studied in the temperature range of 353-413 K and the reaction time range of (please refer to the Supplementary Materials). The apparent activation energy Ea o bifunctional niobium complexes with different halogen ions are shown in Figure 3 order of apparent activation energy Ea is 3a (96.2 kJ/mol) > 3b (68.2 kJ/mol) >3c kJ/mol) (Figure 3), which is consistent with the yields of 96%, 97% and 99% of c carbonate formation from PC and CO2 catalyzed by 3a, 3b, and 3c, respectively. T attributed to the leaving ability of the contained halogen ions (Cl − < Br − < I − ).

Kinetic Study
The kinetics of the carbon dioxide cycloaddition reaction catalyze niobium complexes was also investigated in detail. The cycloaddition o ether (BGE) and CO2 was selected as model reaction at atmospheric p kinetic behavior of bifunctional niobium complexes containing differ was studied in the temperature range of 353-413 K and the reaction tim (please refer to the Supplementary Materials). The apparent activation bifunctional niobium complexes with different halogen ions are shown order of apparent activation energy Ea is 3a (96.2 kJ/mol) > 3b (68.2 kJ/mol) (Figure 3), which is consistent with the yields of 96%, 97% a carbonate formation from PC and CO2 catalyzed by 3a, 3b, and 3c, res attributed to the leaving ability of the contained halogen ions (Cl − < Br − <

Kinetic Study
The kinetics of the carbon dioxide cycloaddition reaction catalyzed by bifuncti niobium complexes was also investigated in detail. The cycloaddition of n-butyl glyc ether (BGE) and CO2 was selected as model reaction at atmospheric pressure, and kinetic behavior of bifunctional niobium complexes containing different halogen was studied in the temperature range of 353-413 K and the reaction time range of 2 (please refer to the Supplementary Materials). The apparent activation energy Ea of bifunctional niobium complexes with different halogen ions are shown in Figure 3. order of apparent activation energy Ea is 3a (96.2 kJ/mol) > 3b (68.2 kJ/mol) >3c ( kJ/mol) (Figure 3), which is consistent with the yields of 96%, 97% and 99% of cy carbonate formation from PC and CO2 catalyzed by 3a, 3b, and 3c, respectively. Th attributed to the leaving ability of the contained halogen ions (Cl − < Br − < I − ). The catalytic performance of the bifunctional niobium complex for various epoxides was also tested at atmospheric pressure and 100 • C ( Table 2, condition b; most boiling points of the substrates studied in our work are above 100 • C except for 2,2-dimethyloxirane (entry 6)). Epichlorohydrin can be converted almost completely within 14 h (entry 1), and for other epoxides with low steric hindrance, relatively good yields were obtained within 24 h ( yields can be achieved within 11 h. All other glycidyl ethers required longer reaction times to achieve excellent yields (entries 9-12).
In summary, the catalytic system has good substrate suitability and can catalyze the reaction of a variety of epoxides with CO 2 to form the corresponding cyclic carbonates under both high pressure and atmospheric conditions with satisfactory results.

Kinetic Study
The kinetics of the carbon dioxide cycloaddition reaction catalyzed by bifunctional niobium complexes was also investigated in detail. The cycloaddition of n-butyl glycidyl ether (BGE) and CO 2 was selected as model reaction at atmospheric pressure, and the kinetic behavior of bifunctional niobium complexes containing different halogen ions was studied in the temperature range of 353-413 K and the reaction time range of 2-8 h (please refer to the Supplementary Materials). The apparent activation energy E a of the bifunctional niobium complexes with different halogen ions are shown in Figure 3. The order of apparent activation energy E a is 3a (96.2 kJ/mol) > 3b (68.2 kJ/mol) >3c (37.4 kJ/mol) (Figure 3), which is consistent with the yields of 96%, 97% and 99% of cyclic carbonate formation from PC and CO 2 catalyzed by 3a, 3b, and 3c, respectively. This is attributed to the leaving ability of the contained halogen ions (Cl − < Br − < I − ).

Reusability of Catalyst
The recyclability of the catalyst was investigated using propylene oxide as a template reaction with CO2 under optimal reaction conditions. After each cycle of reaction, acetone was added to the reaction system to precipitate the catalyst out of the mixture. After filtration and drying under vacuum, the catalyst was reused. The catalyst can be reused at least for five times without obvious loss of catalytic activity and selectivity ( Figure 4). The fresh catalyst and the catalyst after five reactions were selected for IR characterization and the results are shown in Figure 5. The IR spectra indicate that the catalyst was stable even after five cycles. As shown in Figure 5, the typical peaks for the imidazole functionalized complex 3a are visualized clearly and all of them show no significant change before and after five reaction cycles: the peak of 1604 cm −1 is the absorption peak of C=N of Schiff base, the peak of 1601 cm −1 belongs to benzene ring skeleton, and the absorption peaks of 3092 cm −1 and 1172 cm −1 are attributed to the C-H stretching vibration on the imidazole cation and the stretching vibration peak of the imidazole ring, respectively. 80 100 Figure 3. The apparent activation energy E a of the reaction between carbon dioxide and n-butyl glycidyl ether catalyzed by the bifunctional niobium complexes with different halogen ions.

Reusability of Catalyst
The recyclability of the catalyst was investigated using propylene oxide as a template reaction with CO 2 under optimal reaction conditions. After each cycle of reaction, acetone was added to the reaction system to precipitate the catalyst out of the mixture. After filtration and drying under vacuum, the catalyst was reused. The catalyst can be reused at least for five times without obvious loss of catalytic activity and selectivity (Figure 4). The fresh catalyst and the catalyst after five reactions were selected for IR characterization and the results are shown in Figure 5. The IR spectra indicate that the catalyst was stable even after five cycles. As shown in Figure 5, the typical peaks for the imidazole functionalized complex 3a are visualized clearly and all of them show no significant change before and after five reaction cycles: the peak of 1604 cm −1 is the absorption peak of C=N of Schiff base, the peak of 1601 cm −1 belongs to benzene ring skeleton, and the absorption peaks of 3092 cm −1 and 1172 cm −1 are attributed to the C-H stretching vibration on the imidazole cation and the stretching vibration peak of the imidazole ring, respectively. nificant change before and after five reaction cycles: the peak of 1604 cm is the absorp-tion peak of C=N of Schiff base, the peak of 1601 cm −1 belongs to benzene ring skeleton, and the absorption peaks of 3092 cm −1 and 1172 cm −1 are attributed to the C-H stretching vibration on the imidazole cation and the stretching vibration peak of the imidazole ring, respectively.

Possible Reaction Mechanisms
Based on previously reported literature and experimental results [33][34][35][36], a possib reaction mechanism was proposed. As shown in Figure 6, firstly the metal center in th bifunctional catalyst activates the oxygen in the epoxide, then the halogen ion attack nucleophilically the less site-resistant carbon of the epoxide, prompting ring opening the epoxide to form a metal alcoholic salt intermediate, at which point carbon dioxide inserted into the metal alcohol salt intermediate to form a metal carboxylate intermediat and finally the product cyclic carbonate is generated through intramolecular cyclizatio This mechanism suggests that the Lewis acid centers and Lewis base centers play a syn ergistic role in the cycloaddition reaction and are therefore essential in the catalytic sy tem.

Possible Reaction Mechanisms
Based on previously reported literature and experimental results [33][34][35][36], a possible reaction mechanism was proposed. As shown in Figure 6, firstly the metal center in the bifunctional catalyst activates the oxygen in the epoxide, then the halogen ion attacks nucleophilically the less site-resistant carbon of the epoxide, prompting ring opening of the epoxide to form a metal alcoholic salt intermediate, at which point carbon dioxide is inserted into the metal alcohol salt intermediate to form a metal carboxylate intermediate, and finally the product cyclic carbonate is generated through intramolecular cyclization. This mechanism suggests that the Lewis acid centers and Lewis base centers play a synergistic role in the cycloaddition reaction and are therefore essential in the catalytic system. the epoxide to form a metal alcoholic salt intermediate, at which point carbon dioxide is inserted into the metal alcohol salt intermediate to form a metal carboxylate intermediate, and finally the product cyclic carbonate is generated through intramolecular cyclization. This mechanism suggests that the Lewis acid centers and Lewis base centers play a synergistic role in the cycloaddition reaction and are therefore essential in the catalytic system.

Conclusions
In this study, a series of bifunctional niobium complexes were synthesized and characterized by NMR, FTIR spectroscopy and elemental analysis. These catalysts can catalyze the formation of cyclic carbonates from epoxides and CO 2 with high efficiency and selectivity in the absence of solvents and without co-catalysts. By systematic investigation, the optimum reaction conditions were screened as: reaction temperature of 100 • C, carbon dioxide pressure of 1 MPa, reaction time of 2 h and catalyst to epoxide ratio of 1:100. The substrate suitability of the catalysts was studied and the results showed that the catalysts were able to catalyze the cycloaddition of a wide range of epoxides with CO 2 under both high and atmospheric conditions with high selectivity and good to excellent yields. Furthermore, the catalysts showed good recyclability via simple filtration and can be reused for at least five times without obvious loss of catalytic activity and selectivity. A kinetic study of bifunctional niobium complexes containing different halogen ions (3a(Cl − ), 3b(Br − ), 3c(I − )) was carried out and the order of apparent activation energies is 3a (96.2 kJ/mol) > 3b (68.2 kJ/mol) > 3c (37.4 kJ/mol). Finally, a proposed mechanism was given out based on kinetic study and the literature.

Supplementary Materials:
The following supplementary materials can be downloaded at: https://www.mdpi.com/article/10.3390/ma16093531/s1, Table S1: Characterization of intermediates. Table S2.1: The relationship between the yield and time of the reaction of CO 2 and n-butyl glycidyl ether catalyzed by compound 3a under 353 K atmospheric conditions. Table S2.2: The relationship between the yield and time of the reaction of CO 2 and n-butyl glycidyl ether catalyzed by compound 3a under 373 K atmospheric conditions. Table S2.3: The relationship between the yield and time of the reaction of CO 2 and n-butyl glycidyl ether catalyzed by compound 3a under 393 K atmospheric conditions. Table S2.4: The relationship between the yield and time of the reaction of CO 2 and n-butyl glycidyl ether catalyzed by compound 3a under 413 K atmospheric conditions. Figure S1: NMR spectra of the substances synthesized in this work. Figure S2.1: The relationship between the ln(1 − x) and time t of the bifunctional niobium complex 3a catalyzed by the cycloaddition reaction at 353 K-413 K. Table S2.5: The relationship between the yield and time of the reaction of CO 2 and n-butyl glycidyl ether catalyzed by compound 3b under 353 K atmospheric conditions. Table S2.6: The relationship between the yield and time of the reaction of CO 2 and n-butyl glycidyl ether catalyzed by compound 3b under 373 K atmospheric conditions. Table S2.7: The relationship between the yield and time of the reaction of CO 2 and n-butyl glycidyl ether catalyzed by compound 3b under 393 K atmospheric conditions. Table S2.8: The relationship between the yield and time of the reaction of CO 2 and n-butyl glycidyl ether catalyzed by compound 3b under 413 K atmospheric conditions. Figure S2.2: The relationship between the ln(1 − x) and time t of the bifunctional niobium complex 3b catalyzed by the cycloaddition reaction at 353 K-413 K. Table S2.9: The relationship between the yield and time of the reaction of CO 2 and n-butyl glycidyl ether catalyzed by compound 3c under 353 K atmospheric conditions. Table S2.10: The relationship between the yield and time of the reaction of CO 2 and n-butyl glycidyl ether catalyzed by compound 3c under 373 K atmospheric conditions. Table S2.11: The relationship between the yield and time of the reaction of CO 2 and n-butyl glycidyl ether catalyzed by compound 3c under 393 K atmospheric conditions. Figure S2. 3: The relationship between the ln(1 − x) and time t of the bifunctional niobium complex 3c catalyzed by the cycloaddition reaction at 353 K-393 K. Figure S2

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Data Availability Statement:
The data presented in this study are openly available in MDPI.